The present invention relates generally to avionics for aircraft and more specifically to airborne collision avoidance systems and transponders.
Airborne collision avoidance systems provide protection from collisions with other aircraft. Conventional systems operate independently of ground-based air traffic control. The development of an effective airborne collision avoidance system (CAS) has been the goal of the aviation community for many years. In the late 1980""s a system specification for airborne collision avoidance was developed with the cooperation of the airlines, the aviation industry, and the FAA. Systems compliant with this specification became known as the-Traffic Alert and Collision Avoidance System II (herein TCAS). Such systems were mandated by Congress to be installed on most commercial aircraft flying in U.S. airspace by the early 1990""s. A chronology of the development of airborne collision avoidance systems may be found in xe2x80x9cINTRODUCTION TO TCAS IIxe2x80x9d, printed by the Federal Aviation Administration (FAA) of the U.S. Department of Transportation, March 1990, which is incorporated herein by reference.
Conventional TCAS equipment in a host aircraft listens for radio frequency (RF) transmissions from air traffic control transponders on aircraft in its vicinity. Some of the transmissions received by the host aircraft are unsolicited periodic transmissions (squitter) from other aircraft equipped with Mode S transponders; some are replies to interrogation requests made by the host aircraft (surveillance replies); and some are replies to interrogation requests made by other interrogators (aircraft or ground stations) in the vicinity, known as Frequency Replies Unsynchronized In Time (FRUIT). By computer analysis of these transmissions, the airborne TCAS equipment on the host aircraft determines which aircraft are xe2x80x9cintruders,xe2x80x9d in that they may represent a potential collision threat. The TCAS equipment tracks the intruders, and if necessary, provides advisories about intruders to the flight crew to assure separation. Computer analysis of these transmissions can also be employed to passively track intruder aircraft that are too distant to track actively, and therefore not of immediate interest to the collision avoidance function.
TCAS equipment on the host aircraft may either passively track the squitter and FRUIT, or actively track (interrogate) the range of intruder aircraft to elicit a surveillance reply. Whether to commence active tracking of an intruder aircraft depends in part upon information about radio frequency communication reliability. Poor communication may be due to many factors, such as range to the intruder aircraft, RF interference, or shielded antenna paths, and is often associated with low RF signal strength.
In communication technologies (e.g., wired, wireless, radio, or optical) a message is understood by a receiver when it is received in accordance with a signaling protocol intended by the transmitter. Such a protocol may include for example a modulation scheme, a band of radio frequencies, and/or a frequency hopping technique. If the message is received, the transmitter and receiver are generally considered to be using (or sharing) a communication channel, herein also called a link. The link subsumes the signaling protocol and may also include higher level protocols generally associated with computer communications such as acknowledgements and network functions such as sharing of data and processing resources at the transmitter, the receiver, or both. The link may be transitory, persist for an exchange of messages, or.be presumed to be dedicated to particular transmitter(s) and receiver(s). Generally, a link is considered to no longer exist when one or more messages are not received properly; or when there is an indication that one of the transmitter and the receiver may not rely further on the existence of the link for its original purpose.
A conventional TCAS determines whether an RF link is robust enough to attempt acquisition of an intruder aircraft for active tracking, using a link reliability scoring algorithm as follows. Upon detecting an intruder via.receipt of an unsolicited DF=11 squitter, or a DF=17 Automatic Dependent Surveillance-Broadcast (ADS-B) squitter, a link reliability score is initialized for that intruder. Each time that a subsequent squitter or FRUIT is received from the intruder aircraft, the score is incremented by a predetermined, fixed value, which is based upon any previously unsuccessful acquisition attempts of the intruder aircraft. When the link reliability score reaches a predetermined threshold, acquisition of the intruder aircraft is attempted to acquire range information. The number of interrogations made during the acquisition attempt depends upon the number of previously unsuccessful acquisition attempts.
Problems are associated with this method of determining when to commence active tracking of an intruder aircraft. Specifically, this method frequently results in premature attempts to acquire a tracking of the intruder aircraft""s range, and if range is acquired, an accurate tracking of range to the intruder aircraft may be difficult to maintain. That is, the link used by the host aircraft may not be robust enough for the host aircraft to acquire the track, and if the track is acquired, the link may not be robust enough to maintain continuous accurate information about the track. For instance, receipt of as few as three unsolicited replies within a 12-second interval may generally trigger an interrogation-intensive acquisition attempt. If the intruder is distant or the intruder""s antenna is shielded, significant power may be transmitted by the host aircraft trying to acquire track information of an intruder aircraft. In accordance with the TCAS specification, RF transmissions may be subject to a maximum transmitted power during a prescribed time period (e.g., a budget or allocation). Subsequent tracking may consume a significant percentage of the power resources or power allocation available to the aircraft tracking system and may preclude timely acquisition and active tracking of other intruder aircraft using perhaps more reliable RF links.
An improved method of determining when to commence active tracking of an intruder aircraft is desirable. Without such a method, power may be wasted, important links neglected, critical advisories omitted, and hazardous flying conditions develop that may lead to a loss of life and destruction of property.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention may only be gained by taking the entire specification, claims, drawing, and abstract as a whole.
The present invention includes a method for receiving a message or reply from a sender; for determining the signal strength of the message or reply; for determining the time interval that has elapsed since a prior signal from the sender was received; and for determining whether to attempt acquisition of the range of (distance to) the sender in accordance with the signal strength and the time interval measurements.
The present invention includes a system for use in an aircraft, comprising at least one input for accepting antenna signals, and a memory comprising indicia of instructions for performing the method described above.
Premature attempts at such an acquisition are avoided when the determination of whether to attempt an acquisition of a sender""s range is based upon signal strength and upon the time spanning between received squitter. In addition, time, processing resources, and power may be better utilized as a consequence of avoiding maintenance of an unreliable RF link with the sender. Inaccurate communication may also be avoided with consequential improved system performance and reliability.
By employing signal strength and time interval measurement to determine whether to attempt an acquisition of the sender""s range, the system has more resources for transmissions with other senders that are transmitting messages or replies with stronger signal strengths or that are transmitting messages or replies that are more frequently received.
By employing signal strength and time interval measurements to determine whether to attempt an acquisition of the sender""s range, available power is not wasted and power allocations are more effectively utilized.